Presently available data processing systems which are often referred to as belonging to the "mini-computer" class normally handle logical addresses and data words which are 16 bits in length. As used herein, the term "logical" address, sometimes referred to by those in the art as a "virtual" address, is used to denote an address that is programmer visible, an address which the programmer can maipulate. In contrast, a "physical" address is the address of a datum location in the main memory of a data processing system. Operating data processing systems utilize appropriate translation tables for converting logical addresses to physical addresses.
Such mini-computers have been successfully used in many applications and provide a high degree of data processing capability at reasonable cost. Examples of such systems which have found favor in the marketplace are those known as the "Nova" and the "Eclipse" systems designed and developed by Data General Corporation of Westboro, Mass. The Nova and Eclipse family of mini-computers are described in the publications available from Data General Corporation which are listed in Appendix A incorporated as part of this specification.
The Nova system provides a logical address space of 64 kilobytes (the prefix "kilo" more accurately represents 1024, or 2.sup.10) and the Eclipse system also provides a logical address space of 64 kilobytes, both being proven systems for handling many applications at reasonable cost. It is desirable in the development of improved systems to provide for an orderly growth to an even larger logical address space than presently available in Nova and Eclipse systems. Such an extended logical address base permits a larger set of instructions to be utilized by the system, the enlarged instruction set being capable of including substantially all of the basic instructions now presently available in the prior Nova and Eclipse systems as well as a large number of additional, or extended, instructions which take advantage of the increased or expanded logical address space.
Accordingly, such an improved system should be designed to be responsive to software which has been previously designed for use in Nova and Eclipse systems so that those presently having a library of Nova and Eclipse software, representing a substantial investment, can still use such software in the improved, expanded address system. The improving system also would provide for a greater flexibility in performance at a reasonable cost so as to permit more on-line users at a larger number of on-line terminals to utilize the system. The expanded address space would further permit the system to support more extensive and sophisticated programs devised specifically thereafter, as well as to support all of the previous programs supported by the unextended Nova or Eclipse systems.
Moreover, with particular reference to the invention disclosed herein, such system should be designed so that access to different memory locations can be hierarchically arranged, i.e., access to certain memory locations may be permissible to the current user of the system while access to certain other memory locations may be prohibited to the current user of the system to whom such access has not been allocated. Suitable techniques must be devised for controlling such access decisions and for providing appropriate memory location protection checks in order to do so.